Active, or smart, materials are being used increasingly in a variety of industries. Active materials can provide benefits in cost savings over optional apparatus for performing like functions, space, or packaging, savings, and savings of resources such as computer-processing or other system-control resources. The materials can also be referred to as transformable materials because they transform, or change state, when exposed to a specific stimulus, as described further below.
In some cases, active materials allow longer system life, faster performance, smoother actuation, increased reliability, and/or more-accurate performance of the sub-system in which the material is used as compared to optional apparatus.
Active materials are being used in industries including transportation, such as in automotive, aerospace, and marine vehicles. Uses are not limited to transport vehicles, though. Active materials can be used in most any system requiring selective actuation of one or more mechanical components.
An active material can be described also as phase-change material because it performs work by changing its phase in response to being exposed to a specific stimulus, such as heat, electric current, lack of heat (e.g., cold), and radiation.
A popular active material is a shape memory alloy, or SMA. Other exemplary active materials include electroactive polymers (EAPs), piezoelectric materials, magnetostrictive materials, and electrorestrictive materials.
Shape-memory alloy is the generic name given to alloys that exhibit the relatively unusual property of having a strain memory, which can be induced by an input, e.g., a mechanical or thermal input. This unusual property is characterized primarily by two thermo-mechanical responses known as the Shape-Memory Effect (SME) and Superleasticity.
Exemplary alloys include copper alloys (CuAlZn), nickel-titanium-based alloys, such as near-equiatomic NiTi, known as Nitinol, and ternary alloys such as NiTiCu and NiTiNb. A particular exemplary allow includes NiTi-based SMAs. NiTi-based SMAs one or the best, if not the best memory properties—i.e., readily returnable to a default shape, of all the known polycrystalline SMAs. The NiTi family of alloys can withstand large stresses and can recover strains near 8% for low cycle use or up to about 2.5% for high cycle use. The strain recovery capability can enable the design of SMA-actuation devices in apparatuses requiring the selective transfer of torque from a torque generating device to each of a plurality of output shafts.
In an Austenite, or parent phase of an SMA, the SMA is stable at temperatures above a characteristic temperature referred to as the Austenite finish (Af) temperature. At temperatures below a Martensite finish (Mf) temperature, the SMA exists in a lower-modulus phase known as Martensite. The unusual thermo-mechanical response of SMAs is attributed to reversible, solid-state, thermo-elastic transformations between the Austenite and Martensite phases.
Whichever type of active material used, it would be beneficial to determine accurately whether an overheat condition exists at the active material (e.g., SMA wire). Early detection can be used as a trigger to limiting input to the active material, to shutting off input to the active material, providing countering stimulus (e.g., cold), or taking other damage-preventing actions.
One way to determine whether an SMA temperature has surpassed a preferred range is to measure directly a temperature of the SMA or SMA environment—e.g., the environment to which the SMA is exposed. The sensor can send the temperature to a computer which determines whether the SMA is overheating.
Another way to determine whether the SMA temperature has surpassed the preferred range is to position a temperature-sensitive switch on or adjacent the wire. The switch could be, for instance, a thermistor, thermocouple, or resistance temperature detector.
Using such sensors has drawbacks including an increase in required space, or packaging, an increase in system cost, an increase in required resources such as computer-processing, slower system performance, and a possible decrease in system robustness or reliability. As can be seen, many of these shortcomings counter corresponding benefits, mentioned above, of using active materials in the first place.
There is a need for systems and methods configured to accurately determine whether an overheat condition exists for an active material, e.g., SMA wire, during operation of an actuator including the material, and especially for doing so without using a temperature sensor.